View source for Week

{{Short description|Time unit equal to seven days}}
{{Other uses|Week (disambiguation)|Weeks (disambiguation)|Weekly (disambiguation)}}
{{Use dmy dates|date=March 2024}}

A '''week''' is a unit of time equal to seven [[day]]s. It is the standard time period used for short cycles of days in most parts of the world. The days are often used to indicate common work days and rest days, as well as days of [[worship]]. Weeks are often mapped against yearly [[calendar]]s. There are just over 52 weeks in a [[year]]. The term "week" may also be used to refer to a sub-section of the week, such as the [[workweek and weekend]].

Ancient cultures had different "week" lengths, including ten days in Egypt and an eight-day week for [[Etruscan civilization|Etruscans]]. The Etruscan week was adopted by the [[Ancient Rome|ancient Romans]], but they later moved to a seven-day week, which had spread across Western Asia and the Eastern Mediterranean due to the influence of the Christian seven-day week, which is rooted in the [[Jews|Jewish]] seven-day week. In AD 321, Emperor [[Constantine the Great]] officially decreed a seven-day week in the Roman Empire, including making Sunday a public holiday.<ref>{{Cite web |last=Ring |first=Rosanna |date=22 January 2021 |title=A history of time – the story behind our days, weeks, and months |url=https://www.stneotsmuseum.org.uk/articles/a-history-of-time-the-story-behind-our-days-weeks-and-months/ |access-date=6 January 2023 |website=St Neots Museum |language=en-GB}}</ref><ref>[https://www.discovermagazine.com/planet-earth/why-are-there-seven-days-in-a-week Why Are There Seven Days in a Week?]. ''Discover'' (15 January 2020). Retrieved 2022-10-22.</ref> This later spread across Europe, then the rest of the world.

[[File:First Day of Week World Map.svg|thumb|350px|World map showing the first day of the week used in different countries according to the [[Common Locale Data Repository]]<ref>{{Cite web |title=Territory Information |url=https://www.unicode.org/cldr/charts/45/supplemental/territory_information.html |access-date=12 July 2024 |website=www.unicode.org}}</ref>{{legend|#fdc086|Monday}} {{legend|#f0027f|Friday ([[Maldives]] only)}} {{legend|#6fa96f|Saturday}} {{legend|#386cb0|Sunday}}]]

In English, the [[names of the days of the week]] are [[Monday]], [[Tuesday]], [[Wednesday]], [[Thursday]], [[Friday]], [[Saturday]] and [[Sunday]]. In many languages, including English, the days of the week are named after gods or [[classical planet]]s. Saturday has kept its Roman name, while the other six days use Germanic equivalents. Such a week may be called a ''planetary week'' (i.e., a classical planetary week).<ref>{{Cite book|last=Lagasse|first=Paul|url=https://search.credoreference.com/content/entry/columency/week/|title=The Columbia Encyclopedia|publisher=Columbia University Press|year=2018|chapter=Week}}</ref> Certain weeks within a [[year]] may be designated for a particular purpose, such as [[Golden Week (China)|Golden Week in China]] and [[Golden Week (Japan)|Japan]]. More informally, certain groups may advocate [[awareness week]]s, such as [[National Family Week]] in Canada, which are designed to draw attention to a certain subject or cause.

Cultures vary in which days of the week are designated the first and the last, though virtually all have Saturday, Sunday or Monday as the first day. The [[Geneva]]-based ISO standards organization uses Monday as the first day of the week in its [[ISO week date]] system through the international [[ISO 8601]] standard.{{efn|"ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times" is an international standard covering the exchange of date- and time-related data.}} Most of Europe and China consider Monday the first day of the (work) week, while North America, South Asia, and many Catholic and Protestant countries, consider Sunday the first day of the week. It is also the first day of the week in almost all of the Arabic speaking countries.{{Contradictory inline|date=September 2025|reason=The map (sourced from Unicode's CLDR) in the same section gives Saturday as the first day of week in the majority of the Arab world.}} This is culturally and historically the case since in Arabic Sunday is referred to as "Yaom Al'Ahad" which literally means "The first day".<ref name="Days of the week in Arabic Language">{{cite web |title=7 Days of the week in Arabic |url=https://www.toppersmind.com/blog/days-of-week-in-arabic/ |website=ToppersMind |date=6 April 2024 |access-date=23 March 2025}}</ref><ref>{{cite web |last1=Rehberger |first1=Georg |title=What Is the First Day of the Week? |url=https://www.timeanddate.com/calendar/days/first-day-of-the-week.html |publisher=timeanddate.com/ |access-date=23 October 2024}}</ref> Other regions are mixed, but typically observe either Sunday or Monday as the first day.<ref name="firstday">{{Cite web |title=Territory Information |url=https://unicode-org.github.io/cldr-staging/charts/38/supplemental/territory_information.html |access-date=6 November 2020 |website=www.unicode.org}}</ref>

The three [[Abrahamic religions]] observe different days of the week as their holy day. [[Jews]] observe their [[Sabbath]] ([[Shabbat]]) on Saturday, the seventh day, from sundown Friday to sundown Saturday, in honor of God's [[Genesis creation narrative|creation]] of the world in six days and then resting on the seventh.<ref name=":0" /> Most [[Christians]] observe Sunday (the [[Lord's Day]]), the first day of the week in traditional Christian calendars, in honor of the [[resurrection of Jesus]].<ref>{{CCC|pp=2190|end=2191|access-date=25 October 2025|long=on}}</ref> [[Muslims]] observe their [[Sabbath#Islam|"day of congregation"]], known as {{Transliteration|ar|yaum al-[[jumu'ah|jum`ah]]}}, on Friday because it was described as a sacred day of congregational worship in the [[Quran]].<ref>{{cite web |last1=Aslan |first1=Rose |title=What is the significance of Friday prayers in Islam? |url=https://theconversation.com/what-is-the-significance-of-friday-prayers-in-islam-113702 |website=The Conversation |date=19 March 2019 |access-date=10 March 2023}}</ref>

==Name==
{{anchor|Etymology|Names}}
The English word ''[[:wikt:week|week]]'' comes from the [[Old English]] {{lang|ang|wice}}, ultimately from a [[Common Germanic]] ''{{lang|gem-x-proto|wikōn-}}'', from a root ''{{lang|gem-x-proto|wik-}}'' "turn, move, change". The Germanic word probably had a wider meaning prior to the adoption of the [[Roman calendar]], perhaps "succession series", as suggested by [[Gothic language|Gothic]] {{lang|got|wikō}} translating {{lang|grc|taxis}} "order" in [[gospel of Luke|Luke]] 1:8.

The seven-day week is named in many languages by a word derived from "seven". The [[archaism]] '''sennight''' ("seven-night") preserves the old Germanic practice of reckoning time by nights, as in the more common ''[[fortnight]]'' ("fourteen-night").<ref name=worldwidewords>[http://www.worldwidewords.org/weirdwords/ww-sen1.htm sennight] at worldwidewords.org (retrieved 12 January 2017)</ref> '''Hebdomad''' and '''hebdomadal week''' both derive from the [[Koine Greek|Greek]] {{lang|grc|hebdomás}} ({{lang|grc|{{linktext|ἑβδομάς}}}}, "a seven"). '''Septimana''' is cognate with the [[Romance languages|Romance]] terms derived from [[Latin]] {{lang|la|[[wikt:septimana|septimana]]}} ("seven mornings").

==Definition and duration==
A week is defined as an interval of exactly seven [[day]]s,{{efn|1=In pre-modern times, days were measured either from sunset to sunset, or from sunrise to sunrise so that the length of the week (and the day) would be subject to slight variations depending upon the time of year and the observer's geographical latitude.}} so that, except when passing through [[daylight saving time]] transitions or [[leap second]]s,
:1 week = 7 days = 168 hours = 10,080 minutes = 604,800 seconds.
With respect to the [[Gregorian calendar]]:
*1 Gregorian calendar year = 52 weeks + 1 day (2 days in a [[leap year]])
*1 week = {{frac|1600|6957}} ≈ 22.9984% of an average Gregorian month

In a [[Gregorian calendar|Gregorian]] mean year, there are 365.2425 days, and thus exactly {{frac|52|71|400}} or 52.1775 weeks (unlike the [[Julian year (calendar)|Julian year]] of 365.25 days or {{frac|52|5|28}} ≈ 52.1786 weeks). There are exactly 20,871 weeks in 400 Gregorian years, so {{CURRENTDAY}} {{CURRENTMONTHNAME}} {{#expr: {{CURRENTYEAR}} -400}} was a {{CURRENTDAYNAME}} just as was {{CURRENTDAY}} {{CURRENTMONTHNAME}} {{CURRENTYEAR}}.

Relative to the path of the [[Moon]], a week is 23.659% of an average [[lunation]] or 94.637% of an average quarter lunation.

Historically, the system of [[dominical letter]]s (letters A to G identifying the weekday of the first day of a given year) has been used to facilitate [[determination of the day of the week|calculation of the day of week]].
The day of the week can be easily calculated given a date's [[Julian day number]] (JD, i.e. the integer value at [[noon]] [[Universal Time|UT]]):
Adding one to the [[remainder]] after dividing the Julian day number by seven (JD ''[[modulo operation|modulo]]'' 7 + 1) yields that date's [[ISO 8601]] day of the week. For example, the Julian day number of {{CURRENTDAY}} {{CURRENTMONTHNAME}} {{CURRENTYEAR}} is {{#expr: floor({{CURRENTJULIANDAY}}+.5)}}. Calculating {{math|1={{#expr: floor({{CURRENTJULIANDAY}}+.5)}} mod 7 + 1}} yields {{#expr: (floor({{CURRENTJULIANDAY}}+.5) mod 7) + 1}}, corresponding to {{CURRENTDAYNAME}}<!-- +.5 because the JD is integer at noon UT-->.<ref>Richards, E. G. (2013). "Calendars". In S. E. Urban & P. K. Seidelmann, eds. ''Explanatory Supplement to the Astronomical Almanac'', 3rd ed. (pp. 585–624). Mill Valley, Calif.: University Science Books. 2013, pp. 592, 618.
This is equivalent to saying that JD0, i.e. 1 January 4713 BCE of the [[proleptic Julian calendar]], was a Monday.</ref> In 1973, [[John Horton Conway|John Conway]] devised the [[Doomsday rule]] for mental calculation of the weekday of any date in any year.

==Days of the week==
{{Main|Names of the days of the week}}
[[File: Italian - Bracelet - Walters 41269.jpg|thumb|upright=1.75|An Italian [[cameo (carving)|cameo]] bracelet representing the days of the week by their eponymous deities (mid-19th century, [[Walters Art Museum]])]]
[[File: Weekday heptagram.svg|thumb|Schematic comparison of the ordering of the classical planets (arranged in a circle) and the sequence of days in the week (forming a {7/3} [[heptagram]] within the circle).]]

The days of the week were named for the seven [[classical planet]]s, which included the Sun and Moon. This naming system persisted alongside an "ecclesiastical" tradition of numbering the days in [[ecclesiastical Latin]] beginning with ''Dominica'' (the [[Lord's Day]]) as the first day. The Greco-Roman gods associated with the classical planets were rendered in their ''[[interpretatio germanica]]'' at some point during the late Roman Empire, yielding the Germanic tradition of names based on indigenous deities.

The ordering of the weekday names is not the classical order of the planets (by distance in the [[planetary spheres]] model, which is Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn); nor, equivalently, by their apparent speed of movement in the night sky. Instead, the [[planetary hours]] systems resulted in succeeding days being named for planets that are three places apart in their traditional listing. This characteristic was apparently discussed in [[Plutarch]] in a treatise written in c. 100 CE, which is reported to have addressed the question of ''Why are the days named after the planets reckoned in a different order from the actual order?'' (the text of Plutarch's treatise has been lost).<ref>E. G. Richards, ''Mapping Time, the Calendar and History'', Oxford 1999. p. 269.</ref> [[Dio Cassius]] (early 3rd century) gives two explanations in a section of his ''Historia Romana'' after mentioning the Jewish practice of sanctifying the day called the day of [[Cronus|Kronos]] (Saturday).<ref name=Dio>{{cite book |last1=Dio Cassius |title=Ῥωμαϊκὴ Ἱστορία |url=http://www.poesialatina.it/_ns/Greek/testi/Cassius/Historiae_Romanae37.html}} Book 37, Sections 16-19. [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Cassius_Dio/37*.html#note11 English translation].</ref>

{| class="wikitable"
|-
| || [[Sunday]] || [[Monday]] || [[Tuesday]] || [[Wednesday]] || [[Thursday]] || [[Friday]] || [[Saturday]]
|-
|Planet
| [[Planets in astrology#Sun|Sun]]
| [[Moon (astrology)|Moon]]
| [[Mars (astrology)|Mars]]
| [[Mercury (astrology)|Mercury]]
| [[Jupiter (astrology)|Jupiter]]
| [[Venus (astrology)|Venus]]
| [[Saturn (astrology)|Saturn]]
|-
| Greco-Roman deity
| [[Helios]]-[[Sol (Roman mythology)|Sol]]
| [[Selene]]-[[Luna (goddess)|Luna]]
| [[Ares]]-[[Mars (mythology)|Mars]]
| [[Hermes]]-[[Mercury (mythology)|Mercury]]
| [[Zeus]]-[[Iuppiter|Jupiter]]
| [[Aphrodite]]-[[Venus (mythology)|Venus]]
| [[Cronus]]-[[Saturn (mythology)|Saturn]]
|-
| [[Koine Greek|Greek]]:<ref name=Dio/>
| {{lang|grc|ἡμέρα Ἡλίου}}
| {{lang|grc|ἡμέρα Σελήνης}}
| {{lang|grc|ἡμέρα Ἄρεως}}
| {{lang|grc|ἡμέρα Ἑρμοῦ}}
| {{lang|grc|ἡμέρα Διός}}
| {{lang|grc|ἡμέρα Ἀφροδίτης}}
| {{lang|grc|ἡμέρα Κρόνου}}
|-
| [[Classical Latin|Latin]]:
| {{lang|la|cat=0|[[:la:Dies Solis|dies Sōlis]]}}
| {{lang|la|cat=0|[[:la:Dies Lunae|dies Lūnae]]}}
| {{lang|la|cat=0|[[:la:Dies Martis|dies Martis]]}}
| {{lang|la|cat=0|[[:la:Dies Mercurii|dies Mercuriī]]}}
| {{lang|la|cat=0|[[:la:Dies Iovis|dies Iovis]]}}
| {{lang|la|cat=0|[[:la:Dies Veneris|dies Veneris]]}}
| {{lang|la|cat=0|[[:la:Dies Saturni|dies Saturnī]]}}
|-
| {{lang|la|cat=0|[[interpretatio germanica]]}}
| [[Sól (Norse mythology)|Sun]] || [[Máni|Moon]] || [[Týr|Tiwaz]] || [[Wodanaz]] || [[Donar|Þunraz]] || [[Frige]] || — 
|-
| [[Old English]] || {{lang|ang|sunnandæg}} || {{lang|ang|mōnandæg}} || {{lang|ang|tiwesdæg}} || {{lang|ang|wōdnesdæg}} || {{lang|ang|þunresdæg}} || {{lang|ang|frīgedæg}} || {{lang|ang|sæterndæg}}
|-
| [[Indian religions|Indian]] [[Navagraha]] 
| [[Surya|Suryavāra/]] 
[[Ravivar|Ravivāra/Bhānuvāsara/]][[Sunday|Ādityavāra]]
| [[Chandra|Chandravāra/]]
[[Chandra|Somavāra/]]

Induvāsara
| [[Mangala]]vāra/ Bhaumavāsara || [[Budha]]vāra/
Saumyavāsara
| [[Bṛhaspati|Guruvāra]]/[[Bṛhaspati]]vāsara
| [[Shukra]]vāra/ [[Bṛhaspati|Bhṛguvāsara]]|| [[Shani]]vāra/
Sthiravāsara
|}

An ecclesiastical, non-astrological, system of numbering the days of the week was adopted in Late Antiquity. This model also seems to have influenced (presumably via [[Gothic Christianity|Gothic]]) the designation of Wednesday as "mid-week" in [[Old High German]] ({{lang|goh|mittawehha}}) and [[Old Church Slavonic]] ({{lang|cu|срѣда}}, srěda, literally, ''middle day''). Old Church Slavonic may have also modeled the name of Monday, {{lang|cu|понєдѣльникъ}} (literally, ''the day after Sunday''), after the Latin {{lang|la|feria Secunda}}.<ref>Max Vasmer, {{lang|de|Russisches etymologisches Wörterbuch}}, s.v. {{lang|ru|понедельник}}. However, the Slavic languages later introduced a secondary numbering system that names Tuesday as the "second day".<!-- Wiktionary claims "въторъкъ" as OCS, but Vasmer does not recognize an OCS precedent to Russian вторник, so it is unclear when this system was introduced.--></ref>

The ecclesiastical system became prevalent in [[Eastern Christianity]], but in the [[Latin Christianity|Latin West]] it remains extant only in modern [[Icelandic language|Icelandic]], [[Galician language|Galician]], and [[Portuguese language|Portuguese]].<ref>the latter specifically due to the influence of [[Martin of Braga]], 6th-century archbishop of [[Braga]]. {{cite book|author=Richard A. Fletcher|title=The Barbarian Conversion: From Paganism to Christianity|url=https://books.google.com/books?id=RB5aWgr7l-gC&pg=PA257|year=1999|publisher=University of California Press|isbn=978-0-520-21859-8|page=257}}{{cite book|last=McKenna|first=Stephen|title=Paganism and Pagan Survivals in Spain Up to the Fall of the Visigothic Kingdom|year=1938|publisher=Catholic University of America|pages=93–94|url=http://libro.uca.edu/mckenna/paganism.htm|access-date=20 March 2013|chapter=Pagan Survivals in Galicia in the Sixth Century}}</ref>

{| class="wikitable"
|-
|  || "First Day" or<br /> "Lord's Day" <br />([[Sunday]]) || "Second Day" <br />([[Monday]]) || "Third Day" <br />([[Tuesday]]) || "Fourth Day" <br />([[Wednesday]]) || "Fifth Day" <br />([[Thursday]]) || "Sixth Day" <br />([[Friday]]) || "Seventh Day" or<br /> "Sabbath" <br />([[Saturday]])
|-
| [[Byzantine Greek|Greek]]
| {{lang|grc|Κυριακὴ ἡμέρα}}<br/>/kiriaki iméra/
| {{lang|grc|Δευτέρα ἡμέρα}}<br/>/devtéra iméra/ <!--4th century ðéɸteros, 1st century dɛ́ʍtɛros etc., leave the details to Wiktioary-->
| {{lang|grc|Τρίτη ἡμέρα}}<br/>/tríti iméra/
| {{lang|grc|Τετάρτη ἡμέρα}}<br/>/tetárti iméra/
| {{lang|grc|Πέμπτη ἡμέρα}}<br/>/pémpti iméra/
| {{lang|grc|Παρασκευὴ ἡμέρα}}<br/>/paraskevi iméra/<ref>"day of [[wikt:παρασκευή#Ancient Greek|preparation]]", i.e. the day before Sabbath, cf. [[gospel of Luke|Luke]] 23:54 ({{lang|grc|καὶ ἡμέρα ἦν Παρασκευῆς, καὶ σάββατον ἐπέφωσκεν.}})</ref>
| {{lang|grc|Σάββατον}}<br/>/sáb:aton/
|-
| [[Ecclesiastical Latin|Latin]]
| {{lang|la|[dies] dominica}};<br>rarely {{lang|la|feria prima, feria dominica}}
| {{lang|la|feria secunda}}
| {{lang|la|feria tertia}}
| {{lang|la|feria quarta}};<br>rarely {{lang|la|media septimana}}
| {{lang|la|feria quinta}}
| {{lang|la|feria sexta}}
| {{lang|la|Sabbatum; dies sabbatinus, dies Sabbati}};<br> rarely {{lang|la|feria septima, feria Sabbati}}
|-
| [[Hebrew language|Hebrew]]
|{{langx|he|יום ראשון |Yom rishon|first day}}
|{{langx|he|יום שני|Yom sheni|second day}}
|{{langx|he|יום שלישי|Yom shlishi|third day}}
|{{langx|he|יום רביעי|Yom revi'i|fourth day}}
|{{langx|he|יום חמישי|Yom chamishi|fifth day}}
|{{langx|he|יום שישי|Yom shishi|sixth day}}
|{{langx|he|שבת|Shabbat|Rest/cessation}}
|}

==History==
[[File:CLM 14456 71r detail.jpg|thumb|upright=1.75|Circular diagrams showing the division of the day and of the week, from a [[Carolingian Renaissance|Carolingian]] ms. ([[Codex latinus monacensis|Clm]] 14456 fol. 71r) of [[St. Emmeram Abbey]]. The week is divided into seven days, and each day into 24 hours, 96 {{lang|la|puncta}} (quarter-hours), 240 {{lang|la|minuta}} (tenths of an hour) and 960 {{lang|la|cat=no|[[moment (unit)|momenta]]}} (40th parts of an hour).]]

===Ancient Near East===
The earliest evidence of an astrological significance of a seven-day period is a decree of king [[Sargon of Akkad]] around 2300 BCE. Akkadians venerated the number seven, and the key celestial bodies visible to the naked eye numbered seven (the Sun, the Moon and the five closest planets).<ref>{{cite web |title=How we divide time |url=https://www.rmg.co.uk/stories/topics/why-12-months-year-seven-days-week-or-60-minutes-hour |website=[[Royal Museums Greenwich]] |access-date=12 September 2024}}</ref>

[[Gudea]], the priest-king of [[Lagash]] in [[Sumer]] during the [[Gutian dynasty of Sumer|Gutian dynasty]] (about 2100 BCE), built a seven-room temple, which he dedicated with a seven-day festival. In the flood story of the [[Mesopotamia|Assyro-Babylonian]] [[Epic of Gilgamesh]], the storm lasts for seven days, the dove is sent out after seven days (similarly in Genesis), and the [[Noah]]-like character of [[Utnapishtim]] leaves the ark seven days after it reaches the firm ground.{{efn|1=Copeland (1939) states as the date for Gudea "as early as 2600 BCE";<ref>{{cite journal |last=Copeland |first=Leland S. |title=Sources of the Seven-Day Week |journal=Popular Astronomy |date=1939 |volume=47 |issue=4 |page=176 |bibcode=1939PA.....47..175C}}</ref> the modern estimate according to the [[short chronology]] places Gudea in the 22nd century BCE. By contrast, [[Anthony R. Michaelis]] claims that "the first great empire builder, King Sargon I of Akkad ([ruled] 2335 to 2279 BCE [viz., middle chronology]), decreed a seven-day week in his empire. He lived for 56 years, established the first Semitic Dynasty, and defeated the Sumerian City-States. Thus the Akkadian language spread, it was adopted by the Babylonians, and the seven-day week was similarly inherited from him."<ref>{{cite journal |title=The Enigmatic Seven |url=http://garfield.library.upenn.edu/michaelis/title310.pdf |journal=Interdisciplinary Science Reviews |volume=7 |page=373 |last=Michaelis |first=Anthony R.|year=1982 |issue=1 |doi=10.1179/030801882789801278 |bibcode=1982ISRv....7....1M }}</ref> The number seven is significant in Sumerian mythology.<ref>{{cite news |title=The power of seven |date=20 December 2001 |url=https://www.economist.com/christmas-specials/2001/12/20/the-power-of-seven |newspaper=[[The Economist]]}}</ref><!--surely(!) we can come up with a better source for whatever we want to say with this.-->}}

Counting from the [[new moon]], the Babylonians celebrated the 7th, 14th, 21st and 28th of the approximately 29- or 30-day lunar month as "holy days", also called "evil days" (meaning inauspicious for certain activities). On these days, officials were prohibited from various activities and common men were forbidden to "make a wish", and at least the 28th was known as a "rest day".<ref name=ere/>
On each of them, offerings were made to a different god and goddess. Though similar, the later practice of associating days of the week with deities or planets is not due to the Babylonians.<ref>{{cite journal |last1=Emil Schürer |title=Die siebentägige Woche im Gebrauche der christlichen Kirchen der ersten. Jahrhunderte |journal=[[Zeitschrift für die neutestamentliche Wissenschaft]] |date=1905 |pages=1–66 |url=https://dlibra.bibliotekaelblaska.pl/Content/56671/01.pdf |author1-link=Emil Schürer }}</ref>

===Judaism===
A continuous seven-day cycle that runs throughout history without reference to the phases of the moon was first practiced in [[Judaism]], dated to the 6th century BCE at the latest.{{sfnp|Zerubavel|1989|loc=[https://books.google.com/books?id=Cd5ZjRsNj4sC&pg=PA11 p. 11]}}<ref name="Senn 1997"/>

There are several hypotheses concerning the origin of the [[Old Testament|biblical]] seven-day cycle.

[[Friedrich Delitzsch]] and others suggested that the seven-day week being approximately a quarter of a [[lunation]] is the implicit astronomical origin of the seven-day week,<ref name="LSC">{{cite journal
|last=Leland
|first=S. Copeland
|title=Sources of the Seven-Day Week
|journal=Popular Astronomy
|url=http://adsabs.harvard.edu/full/1939PA.....47..175C
|date=April 1939
|volume= XLVII| issue = 4
|page=176 ff
|bibcode=1939PA.....47..175C}}
</ref> and indeed the [[Babylonian calendar]] used [[Intercalation (timekeeping)|intercalary]] days to synchronize the last week of a month with the new moon.<ref>A month consisted of three seven-day weeks and the fourth week of eight or nine days, thus breaking the seven-day cycle every month. Consequently, there is no evidence that the days of the week were given individual names in Babylonian tradition. {{cite book|author=Pinches, T.G.|editor=Hastings, James|others=Selbie, John A., contrib|title=Encyclopedia of Religion and Ethics|volume=20|publisher=Kessinger Publishing|year=2003|pages=889–891|chapter=Sabbath (Babylonian)|url=https://books.google.com/books?id=qVNqXDz4CE8C|isbn=978-0-7661-3698-4 |access-date=17 March 2009}}</ref> According to this theory, the Jewish week was adopted from the Babylonians while removing the moon-dependency.

[[George Aaron Barton]] speculated that the seven-day creation account of Genesis is connected to the Babylonian creation epic, [[Enûma Eliš]], which is recorded on seven tablets.<ref>"Each account is arranged in a series of sevens, the Babylonian in seven tablets, the Hebrew in seven days. Each of them places the creation of man in the sixth division of its series." cited after Albert T. Clay, ''The Origin of Biblical Traditions: Hebrew Legends in Babylonia and Israel'', 1923, [https://books.google.com/books?id=JKBLAwAAQBAJ&pg=PA74 p. 74].</ref>

In a frequently-quoted suggestion going back to the early 20th century,<ref>{{cite book |chapter-url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015027068470;view=1up;seq=197 |chapter=The Babylonian Sabbath |page=181 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015027068470;view=1up;seq=9 |title=The American Antiquarian and Oriental Journal |volume= XXX |year=1908 |access-date=21 June 2018}}</ref> the Hebrew ''[[Biblical Sabbath#Etymology|Sabbath]]'' is compared to the Sumerian ''sa-bat'' "mid-rest", a term for the [[full moon]]. The Sumerian term has been reconstructed as rendered ''Sapattu<sup>m</sup>'' or ''Sabattu<sup>m</sup>'' in [[Babylonian language|Babylonian]], possibly present in the lost fifth tablet of the [[Enûma Eliš]], tentatively reconstructed {{according to whom|date=April 2015}} "[Sa]bbath shalt thou then encounter, mid[month]ly".<!-- [[Akkadian language|Akkadian]] ''um nuh libbi'' ("day of mid-repose") what is this, a reconstruction or an extant translation of Enuma Elish?--><ref name="ere">{{cite book|author=Pinches, T.G.|url=https://books.google.com/books?id=qVNqXDz4CE8C|title=Encyclopedia of Religion and Ethics|publisher=Kessinger Publishing|others=Selbie, John A., contrib|year=2003|isbn=978-0-7661-3698-4|editor=Hastings, James|volume=20|pages=889–891|chapter=Sabbath (Babylonian)|access-date=17 March 2009}}
<!--it turns out this ERE article just summarizes a century-old Assyriologist theory, but we don't know whose it is, or how scholars have judged it--></ref>

However, [[Niels-Erik Andreasen]], [[Jeffrey H. Tigay]], and others claim that the [[Biblical Sabbath]] is mentioned as a day of rest in some of the earliest layers of the [[Pentateuch]] dated to the 9th century BCE at the latest, centuries before the [[Babylonian exile of Judah]]. They also find the resemblance between the Biblical Sabbath and the Babylonian system to be weak. Therefore, they suggest that the seven-day week may reflect an independent [[Israelites|Israelite]] tradition.<ref>{{cite book |url=https://books.google.com/books?id=kE0YAAAAIAAJ |title=The Old Testament Sabbath: A Tradition-historical Investigation |last=Andreasen |first=Niels-Erik A. |date=1972 |publisher=Society of Biblical Literature |isbn=9780891306832 |language=en}}</ref><ref>{{cite journal |last=Shafer |first=Byron E. |date=1974 |title=Reviewed Work: ''The Old Testament Sabbath: A Tradition-Historical Investigation'' by Niels-Erik A. Andreasen |jstor=3263102 |journal=Journal of Biblical Literature |volume=93 |issue=2 |pages=300–301 |doi=10.2307/3263102}}</ref><ref name=":1">{{cite journal |last=Tigay |first=Jeffery H. |date=1998 |title=Shavua |journal=Mo'adei Yisra'el: Time and Holy Days in the Biblical and Second Commonwealth Periods (Heb.), ed. Jacob S. Licht|pages=22–23 }}</ref><ref>{{cite journal |jstor=23506909 |title=New Moons and Sabbaths: A Case-Study in the Contrastive Approach |journal=Hebrew Union College Annual |volume=48 |year=1977 |pages=1–18 |last=Hallo |first=William W.}}</ref> Tigay writes:

<blockquote>It is clear that among neighboring nations that were in position to have an influence over Israel – and in fact which did influence it in various matters – there is no precise parallel to the Israelite Sabbatical week. This leads to the conclusion that the Sabbatical week, which is as unique to Israel as the Sabbath from which it flows, is an independent Israelite creation.<ref name=":1"/><ref>{{cite journal |last=Friedman |first=Allen |date=Sep 2008 |title=Unnatural Time: Its History and Theological Significance |jstor=40914729 |journal=The Torah U-Madda Journal |volume=15 |pages=104–105 |postscript=, Tigay's citation. }}</ref></blockquote>

The seven-day week seems to have been adopted, at different stages, by the [[Persian Empire]], in [[Hellenistic astrology]], and (via [[Hellenistic period|Greek transmission]]) in [[Gupta India]] and [[Tang China]].{{efn|1=It was transmitted to China in the 8th century by Manichaeans, via the country of [[Kangju|Kang]] (a Central Asian polity near [[Samarkand]]).
Tang-era adoption is documented in the writings of the Chinese Buddhist monk [[I Ching (monk)|Yi Jing]] and the Ceylonese Buddhist monk [[Amoghavajra|Bu Kong]].
According to the Chinese encyclopedia ''[[Cihai]]'', there is some evidence that the system had been adopted twice, the first time already in the 4th century ([[Jin dynasty (265–420)|Jin dynasty]]), based on a reference by a Jin-era astrologer, Fan Ning.
The ''Cihai'' under the entry for "seven luminaries calendar" has: <blockquote>method of recording days according to the seven luminaries [{{Lang|zh|七曜}} ''qī yào'']. China normally observes the following order: Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn. Seven days make one week, which is repeated in a cycle. Originated in ancient Babylon (or ancient Egypt according to one theory). Used by the Romans at the time of the 1st century CE, later transmitted to other countries. This method existed in China in the 4th century. It was also transmitted to China by Manichaeans in the 8th century from the country of Kang ({{Lang|zh|康}}) in Central Asia.<ref>{{cite web |title=Japanese Days of the Week: the 'Seven Luminaries' |url=http://www.cjvlang.com/Dow/dowjpn.html |website=Days of the Week in Chinese, Japanese, Vietnamese & Mongolian |publisher=cjvlang}}</ref></blockquote>}}{{citation needed|date=June 2017}}
The Babylonian system was received by the Greeks in the 4th century BCE (notably via [[Eudoxus of Cnidus]]). Although some sources, such as the Encyclopædia Britannica,<ref name=":0">{{cite web |last=Leiman |first=Sid Z. |title=The Sabbath |url=https://www.britannica.com/topic/Jewish-religious-year/The-Sabbath |access-date=28 October 2025 |website=Encyclopædia Britannica |publisher=Encyclopædia Britannica, Inc.}}</ref> state that the Babylonians named the days of the week after the five planets, the sun, and the moon, many scholars disagree. [[Eviatar Zerubavel]] says, "the establishment of a seven-day week based on the regular observance of the Sabbath is a distinctively Jewish contribution to civilization. The choice of the number 7 as the basis for the Jewish week might have had an Assyrian or Babylonian origin, yet it is crucial to remember that the ancient dwellers of Mesopotamia themselves did not have a seven-day week."{{sfnp|Zerubavel|1989|loc=[https://books.google.com/books?id=Cd5ZjRsNj4sC&pg=PA14 p. 9]}} The astrological concept of [[planetary hours]] is an innovation of Hellenistic astrology, probably first conceived in the 2nd century BCE.{{sfnp|Zerubavel|1989|loc=[https://books.google.com/books?id=Cd5ZjRsNj4sC&pg=PA14 p. 14]}}

The seven-day week was widely known throughout the [[Roman Empire]] by the 1st century CE,<ref name=":2">{{cite book |url=https://books.google.com/books?id=PeVLAQAAQBAJ&pg=PA89|title=Written Space in the Latin West, 200 BCE to 300 CE |last1=Keegan |first1=Peter |last2=Sears |first2=Gareth |last3=Laurence |first3=Ray |date=12 September 2013 |publisher=A&C Black |isbn=9781441123046|language=en}}</ref> along with references to the Jewish Sabbath by Roman authors such as [[Seneca the Younger|Seneca]] and [[Ovid]].<ref>{{cite book |url=https://books.google.com/books?id=e3ijDgAAQBAJ&pg=PA21|title=Jesus in Q: The Sabbath and Theology of the Bible and Extracanonical Texts |last=So |first=Ky-Chun |date=6 April 2017 |publisher=Wipf and Stock Publishers |isbn=9781498282116 |language=en}}</ref> When the seven-day week came into use in Rome during the early imperial period, it did not immediately replace the older eight-day [[nundinal cycle|nundinal]] system.<ref>{{cite book |last1=Brind'Amour |first1=Pierre |title=Le calendrier Romain :Recherches chronologiques |date=1983 |publisher=Editions de l'Universitá d'Ottawa |isbn=2760347028 |pages=256–275 |language=fr}}</ref> The nundinal system had probably fallen out of use by the time Emperor [[Constantine the Great|Constantine]] adopted the seven-day week for official use in CE 321, making the [[Sunday|Day of the Sun]] ({{lang|la|dies Solis}}) a legal holiday.<ref>{{cite book |last1=Schaff |first1=Philip |title=History of the Christian Church Vol. III |date=1884 |publisher=T&T Clark |location=Edinburgh |page=[https://archive.org/details/historychristia01schagoog/page/n299 380] |url=https://archive.org/details/historychristia01schagoog|access-date=15 March 2019}}</ref>

===Achaemenid period===
The [[Zoroastrian calendar]] follows the Babylonian in relating the 7th, 14th, 21st, and 28th of the 29- or 30-day lunar month to [[Ahura Mazda]].<ref name="TSZ">Boyce, Mary (ed. & trans.). ''Textual Sources for the Study of Zoroastrianism''. University of Chicago Press, 1984, p. 19-20.</ref>
The forerunner of all modern Zoroastrian calendars is the system used to determine dates in the [[Achaemenid Empire|Persian Empire]], adopted from the Babylonian calendar by the 4th century BCE.

[[Frank Senn|Frank C. Senn]] in his book ''Christian Liturgy: Catholic and Evangelical'' points to data suggesting evidence of an early continuous use of a seven-day week; referring to the Jews during the [[Babylonian captivity]] in the 6th century BCE,<ref name="Senn 1997">{{cite book
| last = Senn
| first = Frank C.
| title = Christian Liturgy: Catholic and Evangelical
| publisher = Fortress Press
| year = 1997
| url = https://books.google.com/books?id=g5c7C2rQzU0C
| isbn = 978-0-8006-2726-3}}</ref> after the destruction of the [[Temple of Solomon]].
While the seven-day week in Judaism is tied to [[Genesis creation narrative|Creation account]] in the [[Book of Genesis]] in the [[Hebrew Bible]], it is not clear whether the Genesis narrative predates the [[Babylonian captivity]] of the Jews in the 6th century BCE. At least since the [[Second Temple Judaism|Second Temple period]] under Persian rule, Judaism relied on the seven-day cycle of recurring [[Sabbath]]s.<ref name="Senn 1997"/>

Tablets{{citation needed|date=April 2015}} from the Achaemenid period indicate that the [[lunation]] of 29 or 30 days basically contained three seven-day weeks, and a final week of eight or nine days inclusive, breaking the continuous seven-day cycle.<ref name=ere/>
The Babylonians additionally celebrated the 19th as a special "evil day", the "day of anger", because it was roughly the 49th day of the (preceding) month, completing a "week of weeks", also with sacrifices and prohibitions.<ref name=ere/>

Difficulties with [[Friedrich Delitzsch]]'s [[origin theory]] connecting Hebrew ''[[Shabbat]]'' with the Babylonian [[lunar cycle]]<ref name=landau>{{cite book
|url=https://archive.org/stream/sabbath00land/sabbath00land_djvu.txt|title=The Sabbath
|author=Landau, Judah Leo
|publisher=Ivri Publishing Society, Ltd
|access-date=26 March 2009
|location=[[Johannesburg, South Africa]]
|pages=2, 12
}}</ref> include reconciling the differences between an unbroken week and a lunar week, and explaining the absence of texts naming the lunar week as ''Shabbat'' in any language.<ref name=orr>{{cite book
|chapter-url=https://books.google.com/books?id=Tn4PAAAAYAAJ&pg=PA2630
|title=The International Standard Bible Encyclopedia
|editor=Orr, James
|editor-link=James Orr (theologian)
|page=2630
|chapter=Sabbath: Critical Theories
|author=Sampey, John Richard
|publisher=Howard-Severance Company
|year=1915}}</ref>

===Hellenistic and Roman era===
{{Main|Nundinae}}

In [[Hellenistic Judaism|Jewish sources]] by the time of the [[Septuagint]], the term "Sabbath" by [[synecdoche]] also came to refer to an entire seven-day week,<ref>''Strong's Concordance'', ''4521''.</ref> the interval between two weekly Sabbaths. [[Parables of Jesus|Jesus's parable]] of the [[Pharisee and the Publican]] describes the Pharisee as fasting "twice in the week". In the account of the women finding the tomb empty, they are described as coming there "toward the one of the sabbaths"; translations substitute "week" for "sabbaths".

The ancient Romans traditionally used the eight-day [[nundinum]] but, after the [[Julian calendar]] had come into effect in 45 BCE, the seven-day week came into increasing use. For a while, the week and the nundinal cycle coexisted, but by the time the week was officially adopted by [[Constantine I (emperor)|Constantine]] in 321 CE, the nundinal cycle had fallen out of use. The association of the [[days of the week]] with the Sun, the Moon and the five planets visible to the naked eye dates to the [[Roman era]] (2nd century).{{sfnp|Zerubavel|1989|loc=[https://books.google.com/books?id=Cd5ZjRsNj4sC&pg=PA45 p. 45]}}<ref name="Senn 1997"/>

The continuous seven-day cycle of the days of the week can be traced back to the reign of [[Augustus]]; the first identifiable date cited complete with [[day of the week]] is 6 February 60 CE, identified as a "[[Sunday]]" (as {{lang|la|viii idus Februarius dies solis}} "eighth day before the ides of February, day of the Sun") in a Pompeiian graffito. According to the (contemporary) Julian calendar, 6 February 60 was, however, a [[Wednesday]]. This is explained by the existence of two conventions of naming days of the weeks based on the [[planetary hours]] system: 6 February was a "Sunday" based on the sunset naming convention, and a "Wednesday" based on the sunrise naming convention.<ref>'' Nerone Caesare Augusto Cosso Lentulo Cossil fil. Cos. VIII idus Febr(u)Arius dies solis, luna XIIIIX nun(dinae) Cumis, V (idus Februaries) nun(dinae) Pompeis''.<br/>{{cite book |author=Robert Hannah |chapter=Time in Written Spaces |editor1=Peter Keegan |editor2=Gareth Sears |editor3=Ray Laurence |title=Written Space in the Latin West, 200 BCE to 300 CE |publisher=A&C Black |year=2013 |page=[https://books.google.com/books?id=PeVLAQAAQBAJ&pg=PA89 89]}}</ref>

===Islamic concept===
According to Islamic beliefs, the seven-day week concept started with the creation of the universe by Allah. [[Abu Hurayra|Abu Huraira]] reported that [[Muhammad]] said: Allah, the Exalted and Glorious, created the clay on Saturday and He created the mountains on Sunday and He created the trees on Monday and He created the things entailing labour on Tuesday and created light on Wednesday and He caused the animals to spread on Thursday and created Adam after 'Asr on Friday; the last creation at the last hour of the hours of Friday, i.e., between afternoon and night.<ref>{{Cite web|title=Sahih Muslim 2789 - Characteristics of the Day of Judgment, Paradise, and Hell - كتاب صفة القيامة والجنة والنار - Sunnah.com - Sayings and Teachings of Prophet Muhammad (صلى الله عليه و سلم)|url=https://sunnah.com/muslim:2789|access-date=17 July 2021|website=sunnah.com}}</ref>

===Adoption in Asia===
====China and Japan====
The earliest known reference in Chinese writings to a seven-day week is attributed to Fan Ning, who lived in the late 4th century in the [[Jin dynasty (265–420)|Jin dynasty]], while diffusions from the [[Manichaeism|Manichaeans]] are documented with the writings of the Chinese Buddhist monk [[I Ching (monk)|Yi Jing]] and the Ceylonese or Central Asian Buddhist monk [[Amoghavajra|Bu Kong]] of the 7th century ([[Tang dynasty]]).

The Chinese variant of the planetary system was brought to Japan by the Japanese monk [[Kūkai]] (9th century). Surviving diaries of the Japanese statesman [[Fujiwara Michinaga]] show the seven-day system in use in [[Heian Period]] Japan as early as 1007. In Japan, the seven-day system was kept in use for astrological purposes until its promotion to a full-fledged Western-style calendrical basis during the [[Meiji Period]] (1868–1912).

====India====
The seven-day week was known in India by the 6th century, referenced in the [[Pancasiddhantika#Pancha-Siddhantika|Pañcasiddhāntikā]].{{Citation needed|date=April 2013}} Shashi (2000) mentions the [[Yuga Purana|Garga Samhita]], which he places in the 1st century BCE or CE, as a possible earlier reference to a seven-day week in India. He concludes "the above references furnish a [[terminus ad quem]] (viz. 1st century) The [[terminus post quem|terminus a quo]] cannot be stated with certainty".<ref>{{cite book
| last = Shashi
| first = Shyam Singh
| title = Encyclopaedia Indica India, Pakistan, Bangladesh Vol. 76 Major dynasties of ancient Orissa: India, Pakistan, Bangladesh
| publisher = Anmol Publications PVT. LTD
| year = 2000
| pages = 114–115
| url = https://books.google.com/books?id=nhYgnKipe-QC
| isbn = 978-81-7041-859-7}}</ref><ref>{{cite book
| author = Pandurang Vaman Kane
| title = [[History of Dharmaśāstra]]
| date = 1930–1962
}}</ref>

===Christian Europe===
{{Further|Holy Week|Easter Week}}

The seven-day weekly cycle has remained unbroken in [[Christendom]], and hence in [[Western history]], for almost two millennia, despite changes to the [[Coptic calendar|Coptic]], [[Julian calendar|Julian]], and [[Gregorian calendar|Gregorian]] calendars, demonstrated by the date of [[Easter]] Sunday having been traced back through numerous [[computus|computistic tables]] to an Ethiopic copy of an early Alexandrian table beginning with the Easter of 311 CE.<ref>{{cite book
| last = Neugebauer
| first = Otto
| title = Ethiopic astronomy and computus
| publisher = Verl. d. Österr. Akad. d. Wiss
| year = 1979
| isbn = 978-3-7001-0289-2}}
</ref><ref>{{cite web |url=http://www.bbc.co.uk/religion/0/20394641 |author=Jayne Lutwyche |date=22 January 2013 |title=Why are there seven days in a week? |publisher=BBC |work=Religion & Ethics |quote=The Roman context of the spread of Christianity meant that Rome contributed a lot to the structure and calendar of the new faith}}</ref>

A tradition of divinations arranged for the days of the week on which certain feast days occur develops in the early medieval period. There are many later variants of this, including the German {{lang|de|Bauern-Praktik}} and the versions of ''[[Erra Pater]]'' published in 16th- to 17th-century England, mocked in [[Samuel Butler (poet)|Samuel Butler]]'s ''[[Hudibras]]''. South and East Slavic versions are known as ''koliadniki'' (from ''koliada'', a loan of Latin {{lang|la|calendae}}), with Bulgarian copies dating from the 13th century, and Serbian versions from the 14th century.<ref name="Ryan380">William Francis Ryan, ''The Bathhouse at Midnight: An Historical Survey of Magic and Divination in Russia'', Penn State Press, 1999
[https://books.google.com/books?id=S3qJMMYH6VYC&pg=PA380 p. 380].</ref>

Medieval Christian traditions associated with the lucky or unlucky nature of certain days of the week survived into the modern period. This concerns primarily [[Friday]], associated with the [[crucifixion of Jesus]]. [[Sunday]], sometimes personified as [[Anastasia the Patrician|Saint Anastasia]], was itself an object of worship in Russia, a practice denounced in a sermon extant in copies going back to the 14th century.<ref name="Ryan383">William Francis Ryan, ''The Bathhouse at Midnight: An Historical Survey of Magic and Divination in Russia'', Penn State Press, 1999
[https://books.google.com/books?id=S3qJMMYH6VYC&pg=PA383 p. 383].</ref>

[[Sunday]], in the ecclesiastical numbering system also counted as the {{lang|la|feria prima}} or the first day of the week; yet, at the same time, figures as the "[[The eighth day (Christian)|eighth day]]", and has occasionally been so called in Christian liturgy.{{efn|1=This is just a reflection of the system of [[ordinal numbers]] in the Greek and Latin languages, where today is the "first" day, tomorrow the "second" day, etc. Compare the [[nundinal cycle]] (literally "nine-days" cycle, describing an [[eight-day week]]) of the Roman calendar, or the [[Resurrection of Jesus]] (after a period of less than 48 hours) being described (in texts derived from Latin) as happening on the "third day".}}

[[Justin Martyr]] wrote: "the first day after the Sabbath, remaining the first of all the days, is called, however, the eighth, according to the number of all the days of the cycle, and [yet] remains the first."<ref>{{cite web |url=http://www.earlychristianwritings.com/text/justinmartyr-dialoguetrypho.html |author=Peter Kirby |title=Saint Justin Martyr: Dialogue with Trypho |website=Early Christian Writings}}</ref>

A period of eight days, usually (but not always, mainly because of Christmas Day) starting and ending on a Sunday, is called an [[octave (liturgy)|octave]], particularly in [[Roman Catholic liturgy]]. In German, the phrase {{lang|de|heute in acht Tagen}} (literally "today in eight days") can also mean one week from today (i.e. on the same weekday). The same is true of the Italian phrase {{lang|it|oggi otto}} (literally "today eight"), the French {{lang|fr|à huitaine}}, and the Spanish {{lang|es|de hoy en ocho}}.

==Numbering==
<!-- This section is linked from [[Week number]] -->
{{Further|Leap week calendar}}

Weeks in a Gregorian calendar year can be numbered for each year. This style of numbering is often used in European and Asian countries. It is less common in the U.S. and elsewhere.

===ISO week date system===
{{More citations needed section|date=June 2020}}

The system for numbering weeks is the [[ISO week date]] system, which is included in [[ISO 8601]]. This system dictates that each week begins on a Monday and is associated with the year that contains that week's Thursday.

====Determining ''Week 1''====
In practice week 1 (''W01'' in ISO notation) of any year can be determined as follows:

* If 1 January falls on a Monday, Tuesday, Wednesday or Thursday, then the week of 1 January is Week 1. Except in the case of 1 January falling on a Monday, this Week 1 includes the last day(s) of the ''previous'' year.
* If 1 January falls on a Friday, Saturday, or Sunday, then 1 January is considered to be part of the last week of the ''previous'' year. Week 1 will begin on the first Monday after 1 January.

Examples:

* Week 1 of 2015 (''2015W01'' in ISO notation) started on Monday, 29 December 2014 and ended on Sunday, 4 January 2015, because 1 January 2015 fell on Thursday.
* Week 1 of 2021 (''2021W01'' in ISO notation) started on Monday, 4 January 2021 and ended on Sunday, 10 January 2021, because 1 January 2021 fell on Friday.

====Week 52 and 53====
It is also possible to determine if the last week of the previous year was Week 52 or Week 53 as follows:

* If 1 January falls on a Friday, then it is part of Week 53 of the previous year (W53-5). 
* If 1 January falls on a Saturday,
** then it is part of Week 53 of the previous year if that is a [[leap year]] (W53-6),
** and part of Week 52 otherwise (W52-6), i.e. if the previous year is a common year. 
* If 1 January falls on a Sunday, then it is part of Week 52 of the previous year (W52-7).

====Schematic representation of [[ISO week date]]====
{| class="wikitable"
|+ [[Dominical letter]](s) plus weekdays, dates and week numbers at the beginning and end of a year
|- valign=bottom
! rowspan=2 | Dominical<br>letter(s)<sup>1</sup>
! colspan=7 | Days at the start of January
! colspan=4 | Effect<sup>1,2</sup>
! colspan=7 | Days at the end of December<sup>1</sup>
|- valign=bottom
! 1<br>Mon || 2<br>Tue || 3<br>Wed ||4<br>Thu || 5<br>Fri || 6<br>Sat || 7<br>Sun
! {{nowrap|W01-1<sup>3</sup>}} !! 01 Jan week !! ... !! 31 Dec week
! 1<br>Mon<sup>4</sup> || 2<br>Tue || 3<br> Wed || 4<br>Thu || 5<br>Fri || 6<br>Sat || 7<br>Sun
|- align=center
! G(F)
| 01 || 02 || 03 || 04 || 05 || 06 || 07
| 01 Jan || W01 || ... || <u>W01</u>
| {{nowrap|31 (30)}} || align=right | (31) ||  ||  ||  ||  ||
|- align=center
! F(E)
|  || 01 || 02 || 03 || 04 || 05 || 06
| <u>31 Dec</u> || W01 || ... || <u>W01</u>
| 30 (29) || {{nowrap|31 (30)}} || align=right | (31) ||  ||  ||  ||
|- align=center
! E(D)
|  ||  || 01 || 02 || 03 || 04 || 05
| <u>30 Dec</u> || W01 || ... || title="W53 when ending year is leap, W01 otherwise." | <u>W01</u> (W53)
| 29 (28) || 30 (29) || {{nowrap|31 (30)}} || align=right | (31) ||  ||  ||
|- align=center
! D(C)
|  ||  ||  || 01 || 02 || 03 || 04
| <u>29 Dec</u> || W01 || ... || W53
| 28 (27) || 29 (28) || 30 (29) || {{nowrap|31 (30)}} || align=right | (31) ||  ||
|- align=center
! C(B)
|  ||  ||  ||  || 01 || 02 || 03
| 04 Jan || <u>W53</u> || ... || W52
| 27 (26) || 28 (27) || 29 (28) || 30 (29) || {{nowrap|31 (30)}} || align=right | (31) ||
|- align=center
! B(A)
|  ||  ||  ||  ||  || 01 || 02
| 03 Jan || title="W53 when ending year is leap, W52 otherwise." | <u>W52</u> (<u>W53</u>)|| ... || W52
| 26 (25) || 27 (26) || 28 (27) || 29 (28) || 30 (29) || {{nowrap|31 (30)}} || style="width:7ex" align=right | (31)
|- align=center
! A(G)
|  || || || || || || 01
| 02 Jan || <u>W52</u> || ... || title="31 December is in W01 only if it is in a leap year."| W52 (<u>W01</u>)
| 25 (31) || 26 (25) || 27 (26) || 28 (27) || 29 (28) || 30 (29) || {{nowrap|31 (30)}} 
|}

'''Notes'''<br />
1. Numbers and letters in parentheses, ( ), apply to March − December in leap years.<br />
2. <u>Underlined</u> numbers and letters belong to previous year or next year.<br />
3. First date of the '''first''' week in the year.<br />
4. First date of the '''last''' week in the year.

===Other week numbering systems===
In some countries, though, the numbering system is different from the ISO standard. At least six numberings are in use:<ref>{{cite web |url=http://www.pjh2.de/datetime/weeknumber/wnd.php?l=en |title=Weeknumber sorted by definition |author=Peter Johann Haas |date=26 January 2002 |website=pjh2.de |access-date=3 April 2015 |archive-date=9 February 2016 |archive-url=https://web.archive.org/web/20160209193500/http://www.pjh2.de/datetime/weeknumber/wnd.php?l=en |url-status=dead }}</ref><ref>{{cite web |url=http://www.merlyn.demon.co.uk/weekinfo.htm |title=Calendar Weeks |publisher=J. R. Stockton |url-status=usurped |archive-url=https://web.archive.org/web/20140113000828/http://www.merlyn.demon.co.uk/weekinfo.htm |archive-date=13 January 2014}}</ref> {{Dubious|date=October 2015}}

{|class="wikitable"
! System
! First day of week
!colspan="3"| First week of year contains
!Can be last week of previous year
! Used by or in
|-
| ISO 8601 || [[Monday]] || {{nowrap|4 January}} || {{nowrap|1st Thursday}} || {{nowrap|4–7 days of year}} || align=center | yes || EU (exc. Portugal) and most other European countries, most of Asia and Oceania
|- 
| Middle Eastern || [[Saturday]] || {{nowrap|1 January}} || {{nowrap|1st Friday}} || {{nowrap|1–7 days of year}} || align=center | yes || Much of the Middle East
|-
| Western traditional || [[Sunday]] || {{nowrap|1 January}} || {{nowrap|1st Saturday}} || {{nowrap|1–7 days of year}} || align=center | yes || Canada, United States, Iceland, Portugal, Japan, Taiwan, Thailand, Hong Kong, Macau, Israel, Egypt, South Africa, the Philippines, and most of Latin America
|-
| [[Broadcast Calendar]] || [[Monday]] || {{nowrap|1 January}} || {{nowrap|1st Sunday}} || {{nowrap|1–7 days of year}} || align=center | yes || Broadcast services in the United States<ref>{{Cite web|title=Broadcast Calendars {{!}} RAB.com|url=http://www.rab.com/public/reports/broadcastCalendar.cfm?type=nm|access-date=26 May 2021|website=www.rab.com}}</ref>
|}

Because the week starts on either Saturday, Sunday, or Monday in all these systems, the days in a [[workweek]], Monday through Friday, will always have the same week number within a calendar week system. Quite often, these systems will agree on the week number for each day in a workweek:

* In years where 1 January is a [[Common year starting on Monday|Monday]], [[Common year starting on Tuesday|Tuesday]], [[Common year starting on Wednesday|Wednesday]], or [[Common year starting on Thursday|Thursday]], all of the above week numbering systems will agree. 
* [[Common year starting on Friday|In years where 1 January is a Friday]], ISO-8601 will be different, but the rest will agree. 
* [[Common year starting on Saturday|In years where 1 January is a Saturday]], ISO-8601 and the Middle Eastern system will agree, being different from Western Traditional and the Broadcast Calendar which will agree. 
* [[Common year starting on Sunday|In years where 1 January is a Sunday]], the Broadcast Calendar will be different, but the rest will agree.

Note that this agreement occurs only for the week number of each day in a work week, not for the day number within the week, nor the week number of the weekends.

{{anchor|Epidemiology}}The '''epi week''' ('''epidemiological week''') is used to report healthcare statistics, as with COVID-19 cases:<ref>{{cite journal |title=Norms and Standards in Epidemiology: Epidemiological Calendar 2000 |url=https://www3.paho.org/english/sha/be993calend.htm |issn=0256-1859 |journal=Epidemiological Bulletin |volume=20 |number=3 |date=September 1999 |access-date=2024-09-11 |publisher=[[Pan American Health Organization]]}}</ref>
<blockquote>The epidemiological week begins on Sunday and ends on Saturday. The first epidemiological week of the year ends on the first Saturday of January, provided that it falls at least four or more days into the month. Therefore, the first epidemiological week may actually begin in December of the previous year.</blockquote>

====Uses====
The [[semiconductor package#Date code|semiconductor package date code]] is often a 4 digit date code YYWW where the first two digits YY are the last 2 digits of the calendar year and the last two digits WW are the two-digit week number.<ref>{{cite web |publisher=Texas Instrument |url=http://focus.ti.com/quality/docs/gencontent.tsp?templateId=5909&navigationId=12626&contentId=153966 |title=Quality & Lead-free (Pb-free): Marking Convention |url-status=dead |archive-url=https://web.archive.org/web/20140405090712/http://focus.ti.com/quality/docs/gencontent.tsp?templateId=5909&navigationId=12626&contentId=153966 |archive-date=5 April 2014}}</ref><ref>{{cite web |publisher=Fairchild Semiconductor |url=http://www.fairchildsemi.com/support/packaging/topmark/dateCode4/ |title=Top Mark Convention – 4-Digit Date Code |url-status=dead |archive-url=https://web.archive.org/web/20140714111530/http://www.fairchildsemi.com/support/packaging/topmark/dateCode4/ |archive-date=14 July 2014}}</ref>

The [[tire code#DOT code|tire date code mandated by the US DOT]] is a 4 digit date code WWYY with two digits of the week number WW followed by the last two digits of the calendar year YY.<ref>{{cite web |url=https://www.law.cornell.edu/cfr/text/49/574.5 |title=49 CFR 574.5 – Tire identification requirements |website=Legal Information Institute}}</ref>

=="Weeks" in other calendars==
The term "week" is sometimes expanded to refer to other time units comprising a few days. Such "weeks" of between four and ten days have been used historically in various places.<ref>[[OED]] s.v. "week ''n.''", entry 1.c.: "Sometimes applied ''transf.'' to other artificial cycles of a few days that have been employed various peoples"</ref> Intervals longer than 10 days are not usually termed "weeks" as they are closer in length to the [[fortnight]] or the [[month]] than to the seven-day week.

===Pre-modern===
Calendars unrelated to the Chaldean, Hellenistic, Christian, or Jewish traditions often have time cycles between the day and the month of varying lengths, sometimes also called "weeks".

An [[eight-day week]] was used in [[Roman calendar#Nundinal cycle|Ancient Rome]] and possibly in the pre-Christian [[Celtic calendar]]. Traces of a nine-day week are found in Baltic languages and in [[Welsh language|Welsh]]. The ancient [[Chinese calendar]] had a ten-day week, as did the ancient [[Egyptian calendar]] (and, incidentally, the [[French Republican Calendar]], dividing its 30-day months into thirds).

A six-day week was used in the [[Akan calendar]] and [[Kabye people|Kabiye]] culture in [[West Africa]] until 1981. Several cultures used a five-day week, including the [[Javanese calendar]] and the traditional cycle of market days in [[Korean culture|Korea]].{{Citation needed|date=July 2015}} The [[Igbo Culture#Calendar (Iguafo Igbo)|Igbo]] have a "market week" of four days. Evidence of a "three-day week" has been derived from the names of the days of the week in [[Guipuscoan Basque]].<ref name="Knorr">''[http://dialnet.unirioja.es/servlet/fichero_articulo?articulo=26362&orden=59718 Astronomy and Basque Language]'', [[Henrike Knörr]], ''Oxford VI and SEAC 99 "Astronomy and Cultural Diversity"'', [[San Cristóbal de La Laguna|La Laguna]], June 1999. It references [[Alessandro Bausani]], 1982, ''The prehistoric Basque week of three days: archaeoastronomical notes'', ''The Bulletin of the [[Center for Archaeoastronomy]]'' (Maryland), v. 2, 16–22.
1. ''astelehena'' ("week-first", Monday), 2. ''asteartea'' ("week-between", Tuesday), 3. ''asteazkena'' ("week-last", Wednesday).</ref>

The Aztecs and Mayas used the [[Mesoamerican calendars]]. The most important of these calendars divided a ritual cycle of 260 days (known as ''[[Tonalpohualli]]'' in [[Nahuatl]] and ''[[Tzolk'in]]'' in [[Yucatec Maya]]) into 20 weeks of 13 days (known in Spanish as [[trecena]]s). They also divided the solar year into 18 periods (''winal'') of 20 days and five nameless days (''wayebʼ''), creating a 20-day month divided into four five-day weeks. The end of each five-day week was a market day.{{sfnp|Zerubavel|1989|loc=[https://books.google.com/books?id=Cd5ZjRsNj4sC&pg=PA50 pp. 50–54]}}<ref>{{cite web |url=http://www.aztec-history.com/aztec-calendar-stone.html |title=Aztec calendar stone |website=aztec-history.com}}</ref>

The Balinese [[Pawukon]] is a 210-day calendar consisting of 10 different simultaneously running weeks of 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 days, of which the weeks of 4, 8, and 9 days are interrupted to fit into the 210-day cycle.

===Modern reforms===
{{Further|International Fixed Calendar|Decimal calendar|French Revolutionary Calendar#Ten days of the week|Bahá'í calendar}}

The [[International Fixed Calendar]] (also known as the "Eastman plan") kept a 7-day week while defining a year of 13&nbsp;months with 28&nbsp;days each (364&nbsp;days). Every calendar date was always on the same weekday. It was the official calendar of the [[Eastman Kodak Company]] for decades.{{Citation needed|date=September 2025}}

A 10-day week, called a [[French Revolutionary Calendar#Ten days of the week|''décade'']], was used in Revolutionary France for nine and a half years from October&nbsp;1793 to April&nbsp;1802.<ref>{{cite book |author=Antoine Augustin Renouard |title=Manuel pour la concordance des calendriers républicain et grégorien |url=https://books.google.com/books?id=oUoMAAAAIAAJ |access-date=14 September 2009 |edition=2 |date=1822 |publisher=A. A. Renouard}}</ref> The [[Paris Commune]] adopted this calendar for 18&nbsp;days in 1871.

The [[Bahá'í calendar]] features a 19-day period that some classify as a month and others classify as a week.<ref>{{cite book |title=The Seven-Day Circle |last=Zerubavel |first=Eviatar |publisher=The Free Press |year=1985 |isbn=0029346800 |location=New York |pages=[https://archive.org/details/sevendaycircleth00zeru/page/48 48–50] |author-link=Eviatar Zerubavel |url=https://archive.org/details/sevendaycircleth00zeru/page/48 }}</ref>

====Soviet====
{{Further|Soviet calendar}}
{{Multiple image
| align = right
| direction = horizontal
| width = 150
| image1   = Soviet calendar 1930 color.jpg
| caption1 = Soviet calendar, 1930.<br/>Five colors of five-day work week repeat.
| image2   = Soviet calendar 1933 color.jpg
| caption2 = Soviet calendar, 1933.<br/>Rest day of six-day work week in blue.
| footer   = Days of each Gregorian month in both calendars are grouped vertically into seven-day weeks.
}}

In the [[Soviet Union]] between 1929 and 1940, most factory and enterprise workers, but not collective farm workers, used five and six&nbsp;day work weeks while the country as a whole continued to use the traditional seven&nbsp;day week.<ref name=Foss>{{cite journal |first=Clive |last=Foss |title=Stalin's topsy-turvy work week |journal=History Today |volume=54 |issue=9 |date=September 2004 |pages=46–47}}</ref><ref name=Russie>{{cite web |website=iCalendrier |url=http://icalendrier.fr/calendriers-saga/etudes-thematiques/reforme-gregorienne#russie |title=La réforme en Russie: Il faudra attendre ... plus de trois siècles |language=fr |trans-title=The reform in Russia: It will be necessary to wait ... more than three centuries}}</ref><ref>{{cite book |author-link=Eviatar Zerubavel |last=Zerubavel |first=Eviatar |chapter=The Soviet five-day ''Nepreryvka'' |title=The Seven Day Circle |isbn=0029346800 |location=New York |publisher=Free Press |year=1985 |pages=[https://archive.org/details/sevendaycircleth00zeru/page/35 35–43] |chapter-url=https://archive.org/details/sevendaycircleth00zeru/page/35 }}</ref>

From 1929 to 1951, five national holidays were days of rest ({{nowrap|22 January}}, {{nowrap|1–2 May}}, {{nowrap|7–8 November}}). From autumn&nbsp;1929 to summer&nbsp;1931, the remaining 360&nbsp;days of the year were subdivided into 72&nbsp;five&nbsp;day work weeks beginning on {{nowrap|1 January}}. Workers were assigned any one of the five days as their day off, even if their spouse or friends might be assigned a different day off. Peak use of the five&nbsp;day work week occurred on {{nowrap|1 October 1930}} at 72% of industrial workers. From summer&nbsp;1931 until {{nowrap|26 June 1940}}, each Gregorian month was subdivided into five six&nbsp;day work weeks, more-or-less, beginning with the first day of each month. The sixth day of each six&nbsp;day work week was a uniform day of rest. On {{nowrap|1 July 1935}} 74.2% of industrial workers were on non-continuous schedules, mostly six&nbsp;day work weeks, while 25.8% were still on continuous schedules, mostly five&nbsp;day work weeks. The Gregorian calendar with its irregular month lengths and the traditional seven&nbsp;day week were used in the Soviet Union during its entire existence, including 1929–1940; for example, in the masthead of ''[[Pravda]]'', the official Communist newspaper, and in both Soviet calendars displayed here. The traditional names of the seven&nbsp;day week continued to be used, including "Resurrection" (Воскресенье) for Sunday and "Sabbath" (Суббота) for Saturday, despite the government's [[Marxist–Leninist atheism|official]] [[state atheism|atheism]].

==See also==
{{Commons}}
*[[Determination of the day of the week]]
*[[GPS week number]]
*[[Names of the days of the week]]
*[[Workweek and weekend]]

==Notes==
{{Notelist}}

==References==
{{Reflist}}
*{{cite book |last=Zerubavel |first=Eviatar |title=The Seven Day Circle: The History and Meaning of the Week |date=1989 |url=https://books.google.com/books?id=Cd5ZjRsNj4sC |publisher=University of Chicago Press |isbn=978-0-226-98165-9}}

==Further reading==
*{{cite book
| last = Colson
| first = Francis Henry
| title = The Week: An Essay on the Origin and Development of the Seven-day Cycle
| publisher = Cambridge University Press
| year = 1926
| oclc = 59110177}}
*{{cite EB1911|wstitle=Week |volume=28|page=466|last= Thomas |first= Northcote Whitridge |author-link= Northcote Whitridge Thomas}}

{{Time topics}}
{{Time measurement and standards}}
{{Authority control}}

[[Category:Weeks| ]]
[[Category:Units of time]]